The present invention relates to an electrochemical method and apparatus for purifying fluids, especially waste water or air, e.g. for the purposes of decontamination and/or disinfection. Current methods for treating organic, inorganic and microbiological pollutants in air, water and soil include so-called xe2x80x9cadvanced oxidation processesxe2x80x9d. These advanced oxidation processes create a highly reactive oxidizing agent that can readily convert organic waste to carbon dioxide and water, while mineralizing inorganic constituents for easy removal. One of the agents considered in this context is the OH radical, which can be created by the Fenton reaction
Fe2++H2O2+H+xe2x86x92Fe3++.OH+H2O.xe2x80x83xe2x80x83(1)
The Fe3+-ion is subsequently reduced by hydrogen peroxide via the reaction
Fe3++H2O2xe2x86x92Fe2++.O2H+H+xe2x80x83xe2x80x83(2)
Various ways of providing H2O2 and Fe2+ for process (1) were proposed in the past.
J. S. Do and C. P. Chen, J. Electrochem. Soc. 140 (1993), 1632 proposed to create H2O2 by the reaction
O2+2H++2exe2x88x92xe2x86x92H2O2
on a graphite, reticulated carbon or carbon-PTFE cathode. Fe2+ was added externally.
K. Pratap, A. T. Lemley, J. Agric. Food Chem 42 (1994), 209 proposed to generate Fe2+ by a sacrificial iron anode and to add H2O2 externally.
It was also proposed by E. Brillas et al., Electrochem. Solid-State Lett. 1 (1998), 168 to combine these two approaches, i.e. to produce H2O2 on a carbon cathode, as indicated above, and to generate Fe2+ by a sacrificial iron anode.
EP 0 694 501 A1 discloses an electrochemical cell for purification of contaminated water using OH radicals. In this document it is proposed to enable the Fenton reaction by adding FeSO4 to a liquid electrolyte. Although this electrochemical cell was shown to be effective in destroying pollutants, it requires the continuous addition of FeSO4 when purifying a continuous fluid flow. Furthermore, the classical Fenton reaction, as employed according to this document, has several intrinsic limitations in that it requires H2O2 to be created, which consumes hydroxyl radicals. Furthermore, the regeneration of Fe2+ according to reaction (2) is relatively slow so that Fe3+ accumulates in the system, which slows down the reaction rate and leads to precipitation of colloidal solids containing Fe(OH)3, the so-called Fenton-sludge.
U.S. Pat. No. 5,645,700 discloses an electrolytic cell for generation of hydrogen peroxide, wherein a catalyst for a reaction for creating hydrogen peroxide is used which may be a complex of group VIII metals with porphyrines and phthalocyanines as ligands. These ligands prohibit the creation of OH radicals.
John E. Biaglov and Alexander V. Kachur, Radiation Research 148 (1997), 181 and Alexander V. Kachur, Stephen W. Tuttle and John E. Biaglov, Radiation Research 150 (1998), 475 consider the role of Fe-complexes in creating the hydroxyl radical in biological system. They found that a class of Fe-complexes having polyphosphates or acetic derivatives of ethyleneamine as ligands promote the generation of the OH radical. These complexes were considered for modelling biological processes. A relation of this effect to the purification of fluids, for example waste water, was not considered.
All previously mentioned Fenton-type processes require the addition of an iron salt or an Fe complex to a liquid, or the provision of a sacrificial iron anode.
It is the object of the present invention to provide a basically self-sustained apparatus for purifying fluids employing an electrochemical cell and a related method for purifying fluids.
This object is accomplished according to the invention by an apparatus for purifying fluids comprising at least one electrochemical cell having a cathode, an anode and an electrolyte between cathode and anode, said cell comprising a metal complex, ML, immobilized at or in a solid at the cathode side of the electrolyte, as opposed to the anode side, wherein M represents a metal, L represents an organic or inorganic ligand, said complex being capable of forming the hydroxyl radical by a reaction wherein the metal in the complex is oxidised and acquires an additional positive charge, said anode creating positive ions and electrons, said electrolyte allowing the transfer of positive charges, said solid being arranged such that the fluid to be purified can come into contact with the metal complex.
Said oxidation of said metal may especially involve oxygen and/or H+ ions
According to a preferred embodiment, the metal complex is an iron complex, FeL, wherein the FeL complex may allow for the iron having two different oxidation states Fe2+L and Fe3+L.
The solid may especially be the cathode or the electrolyte. According to the invention, the metal complex may be deposited, attached or incorporated into the cathode or into a solid electrolyte at the cathode side thereof. In the latter case, the cathode may be bonded, deposited or otherwise attached to said cathode side of the electrolyte in a way allowing the reduction of the metal in the complex by providing an electron, e.g. from Fe3+ to Fe2+.
The anode and cathode are connected or connectable to a power source which may or may not be part of the apparatus.
Usually, the apparatus according to the invention will comprise means for guiding the fluid to be purified to the cathode and/or means for guiding purified fluid away from the cathode.
The invention may also provide that said Fe complex FeL is capable of undergoing the net reaction
3Fe2+L+O2+3H+xe2x86x923Fe3+L+.OH+H2O.xe2x80x83xe2x80x83(3)
Fe complexes enabling other reactions involving agents that readily produce oxygen, e.g. H2O2,.O2H etc., may also be considered in order to produce the hydroxyl radical. Especially, the Fe complex may be capable of undergoing the reaction.
Fe2+L+H2O2+H+xe2x86x92FeL3++.OH+H2O,
wherein H2O2 may be externally provided from an H2O2 source or produced in situ.
Preferably said anode enables a reaction creating H+ ions and electrons and said electrolyte allows the migration of H+ ions, although other ways of providing H+ ions may be contemplated.
The invention may also provide that said anode comprises a catalyst for promoting the reaction.
H2Oxe2x86x92xc2xdO2+2H++2exe2x88x92.xe2x80x83xe2x80x83(4)
Said catalyst may be a metal oxide or a mixture of metal oxides containing at least one transition metal and often including one of the precious metals, such as Ru, Rh, Pd, Os, Ir, Pt.
Said catalyst may especially be ruthenium dioxide, iridium dioxide and mixed oxides of ruthenium and manganese, ruthenium and titanium, or nickel and cobalt.
Chelating agents suitable for forming a Fe complex are generally suitable ligands L.
The invention may provide that the ligand L is selected from the group comprising polyphosphates, pyrophosphates, bisphosphonates, polyaminocarboxylates, citrates, ethylene amines and derivatives thereof and further comprising analogues of said substances and derivatives which allow covalent attachment to the cathode.
The ethyleneamines mentioned above especially comprise ethylenediamine and ethylenediaminetetraacetic acid (EDTA).
The invention may also provide that said ligand is selected from the group comprising acetic derivatives of ethylene amine, polyphosphates or bisphosphonates and analogues thereof.
The ligand may be coupled directly to the surface of the electrode via a functional moiety selected from the group comprising an amino group, a hydroxyl group and a carbonyl group. In one embodiment the amino group may form part of 3-aminopropylsilanol or of derivatives thereof, wherein the attachment to the electrode surface occurs through the silanol moiety.
The invention may especially provide that the ligands are of the general form
Rxe2x80x94(NX2)p,
wherein X is selected from the group comprising 
and wherein 1xe2x89xa6pxe2x89xa62;
and wherein R is an organic moiety.
R is preferably selected from the group comprising
alkylsilanol, the preferred alkyl being C1-C10 alkyl, especially propyl,
substituted and unsubstituted triazines, triazine being preferably substituted in the 2-, 4- or 6-position with n-amino-(CH2)n silanol, n being an integer, preferably 1xe2x89xa6nxe2x89xa610, more preferably n=3, and
[(CH2)m(NX)]q(CH2)n(NX)(CH2)oSiO3,
xe2x80x83o being an integer, preferably 3, n being an integer, preferably 2, q being an integer, preferably selected from the range of 0xe2x89xa6qxe2x89xa65, m being an integer, preferably 2.
Other ligands that may be contemplated are chelating resins containing iminodiacetic or aminoalkylphosphonic acid groups, especially those disclosed in U.S. Pat. No. 5,804,606 to Surowiec et al., to which reference is made for further details.
If the substrate to which the ligand is attached is a carbon based substrate, covalent attachment of the ligand may occur through an amino group or by an oxygen-containing functional group such as a carbonyl group or a hydroxyl group, especially a hydroxyl group, wherein a hydrogen atom is substituted by said ligand. If the substrate is a metal oxide, covalent attachment by silylation is preferred.
According to the invention, the ligand may be attached to the cathode in a covalent manner.
The invention may provide that the ligand exhibits a covalent bond to the cathode obtainable by silylation.
The ligand may especially be attached to the cathode by a silyl group or silyl groups.
The invention may also provide that said anode and/or cathode is a carbon or metal oxide electrode.
The invention may also provide that said anode and/or cathode is porous.
The invention may also provide that it comprises means for conducting the fluid to be purified to a side of the cathode where it may come into contact with the Fe-complex, said anode being isolated from the fluid to be purified.
The invention may also provide that the electrolyte is a solid electrolyte membrane with the cathode and anode attached, deposited or otherwise connected thereto. According to a specific embodiment, the metal complex ML and/or the catalyst for promoting reaction (4) are incorporated in the electrolyte at the anode and cathode side thereof respectively.
The invention may also provide that the electrolyte is sandwiched between a cathode and an anode, said cathode having a side contacting the fluid to be purified and an electrolyte contacting side, and that the electrolyte prohibits the migration of the fluid to be purified and/or of pollutants therein to the anode.
Especially in this embodiment porous electrodes may be provided.
The invention may also provide that said electrolyte allows the migration of O2 and H2O.
The invention may also provide that the apparatus comprises a plurality of electrochemical cells and means for bringing the fluid to be purified in contact with the cathode of each cell.
The invention may also provide that said plurality of cells is arranged in a series, parallel and/or cyclic configuration with respect to the flow of the fluid to be purified.
The invention may provide that at least part of the outer walls of a cell form an electrode of said electrochemical cell and at least part of the cells share a common wall with another cell, said common wall forming an electrode for at least two cells.
The invention may also provide that the apparatus comprises conduit means for conducting the fluid to be purified, at least part of the walls of said conduit means and preferably the entire walls being formed by the cathode of one or more electrochemical cells.
The invention may especially provide that the electrochemical cell comprises a closed channel for conducting the fluid to be purified therethrough, at least part of the walls of said channel being formed by the cathode of said electrochemical cell.
The invention may also provide an electrochemical cell for use in an apparatus or method for purifying fluids, especially waste water, comprising a cathode, an anode and an electrolyte, said cell comprising a metal complex, especially an Fe-complex, FeL, immobilized at or in a solid at the cathode side of the electrolyte, as opposed to the anode side, said complex comprising an organic or inorganic ligand L, said complex being capable of forming the hydroxyl radical by a reaction wherein the metal in the complex is oxidised and acquires an additional positive charge, said anode enabling a reaction creating positive ions and electrons, said electrolyte allowing the transfer of positive charges. Said positive ion may especially be a H+ ion. In order to complete the reaction cycle, said metal, having been oxidised or being in an oxidised state, is electrochemically reduced.
The invention may also provide an electrode for use in an electrochemical apparatus for purifying fluids, wherein said electrode comprises a metal complex, especially an Fe-complex, FeL, said complex comprising an organic or inorganic ligand L, said complex being capable of forming the hydroxyl radical by a reaction wherein the metal in the complex is oxidised and acquires an additional positive charge.
More specific embodiments of the electrochemical cell and the electrode according to the invention have already been outlined above.
The invention may also provide a method of purifying fluids, especially waste water, comprising the steps of providing a cathode and an anode, said anode and cathode being separated by an electrolyte allowing the transfer of positive charges, and providing a metal complex, especially an Fe-complex, FeL, immobilized at or in a solid at the cathode side of the electrolyte, said complex comprising an organic or inorganic ligand L, said complex being capable of forming the hydroxyl radical by a reaction wherein the metal in the complex is oxidised and acquires an additional positive charge, said anode creating positive ions and electrons, connecting said cathode and anode to an electric power source, conducting fluid to be purified to the metal complex.
The invention may also provide that said Fe complex FeL is capable of undergoing the reaction (3)
3Fe2+L+O2+3H+xe2x86x923Fe3+L+.OH+H2O,
said anode enabling a reaction creating H+ ions and electrons, said anode and cathode being separated by an electrolyte, said electrolyte allowing the migration of H+-ions. The anode may also comprise a catalyst for promoting reaction (4).
The invention may also provide that O2 is conducted either separately or together with said fluid to be purified to the cathode or the cathode side of the electrolyte to come into contact with the Fe-complex.
The invention may also provide that O2 is added to the fluid to be purified.
The invention may also provide that said electrolyte allows the transfer of O2 and that O2 is fed to said electrolyte.
The invention may also provide that H2O is supplied to the anode or to the anode side of the electrolyte to come into contact with the reaction site, especially the catalyst at the anode.
The invention may also provide that said electrolyte allows the transfer of H2O and H2O is fed to said electrolyte.
Whereas specific reference was made previously to a metal complex and especially an Fe complex, the invention more generally provides for a catalyst for promoting an electrochemical reaction creating the hydroxyl radical, said catalyst being immobilized at or in a solid at the cathode side of the electrolyte.
According to the preferred embodiments of the invention, the hydroxyl radical is electrochemically generated by a method comprising:
providing a catalytic O2 generating anode, especially a water oxidising anode.
providing a catalytic cathode generating the hydroxyl radical,
positioning a polymeric electrolyte ion transporting membrane between the anode and the cathode, the anode being bonded to one surface of the membrane and the cathode being bonded to the other surface of the membrane,
providing a direct potential between the anode and cathode and supplying O2 to the cathode.
The apparatus according to the invention has several distinct advantages over the prior art. No external source of H2O2 or elemental Fe is required. The only reagent consumed in the overall process is O2, which is supplied by air. Water can be purified without adjusting its pH or ionic strength. In contrast the prior art requires acidic conditions and high salt concentrations (for adequate ionic conductivity between anode and cathode). Gases (such as air) can also be purified.